The present invention relates to a method and apparatus for repeatedly and automatically threading the wire electrode of an EDM apparatus through a workpiece.
When cutting a workpiece in a traveling wire EDM apparatus, some operations require that the electrode wire be removed from the cut being effected in the workpiece and rethreaded into another starting hole in the workpiece, the workpiece being appropriately positioned prior to each of the rethreading operations. Because the diameter of the electrode wire is very small, for example 250 microns, and is only slightly smaller than the bore diameter of the wire guide members, such bore diameter being, for example, 252 microns, which is a requirement for accurate positioning of the wire by its guide members, threading the wire through such a small bore, only 2 microns in diameter wider than the wire diameter, is a rather difficult accomplishment. This is particularly true if the threading of the wire through the wire guide members is automatic. Because the wire feed mechanism, displacing the wire along its longitudinal axis, is located outside of, and a certain distance away from the wire support and guide "heads", the electrode wire end, during threading and rethreading, must be passed from its free end over a considerable distance through the wire support and guide heads. If the clearance between the wire peripheral surface and the wire support and guide members is relatively narrow, considerable fricion results, and it is quite possible for the end of the electrode wire to hit some of the components in the wire support and guide heads and to become distorted to a point of not being capable of being correctly threaded, in spite of its inherent rigidity.
A few solutions to that problem have been proposed in the past, such as disclosed in Federal Republic of Germany published patent application No. 3,037,505 which discloses forming the electrode wire with a needle point prior to rethreading the wire. Such proposed solution to the problem, however, provides unsatisfactory results, in view of the close tolerance between the bore of the wire guide members and the size of the wire itself, which may lead to considerable resistance to threading the body of the wire through the wire guide members and other elements disposed in the wire support and guide heads.